Conventional multifrequency generators as used in telecommunications are well-known as shown in many publications including U.S. Pat. No. 2,824,173 L. Meacham (Feb. 18, 1958).
In this and subsequent circuits, two of a plurality of resonant circuits were coupled to the outgoing line in response to depression of a push button at the calling instrument.
More recently, a number of patents have shown circuits eliminating the need for resonant circuits. A typical one of these is U.S. Pat. No. 3,941,942 to H. Nash issued Mar. 2, 1976. In this circuit the output of a basic clock frequency is converted to two series of pulses on depression of a button, the two tones being synthesized resistively for transmission.
Another approach is shown in U.S. Pat. No. 3,787,836 to Hagelbarger issued Jan. 22, 1974. In that patent, a single base frequency is produced and fed to two dividers operating in response to operation of the keyboard to clock predetermined sequences of digital signals which are phase shifted in predetermined phase amounts.